Semiconductor device

ABSTRACT

The present invention provides a sense circuit for DRAM memory cell to cover the events that a sense time becomes remarkably longer when a power source voltage is lowered, a sense time under the low voltage condition becomes shorter when temperature rises and a sense time changes to a large extent for fluctuation of processes. The present invention provides the following typical effects A switch means is provided between the bit line BL and local bit line LBL connected to the memory cells for isolation and coupling of these bit lines The bit line BL is precharged to the voltage of VDL/2, while the local bit line LBL is precharged to the voltage of VDL. The VDL is the maximum amplitude voltage of the bit line BL A sense amplifier SA comprises a first circuit including a differential MOS pair having the gate connected to the bit line BL and a second circuit connected to the local bit line LBL for full amplitude amplification and for holding the data. When the bit line BL and local bit line LBL are capacitance-coupled via a capacitor, it is recommended to use a latch type sense amplifier SA connected to the local bit line LBL.

TECHNICAL FIELD

The present invention relates to a semiconductor device and particularlyto a semiconductor integrated circuit device having excellent lowvoltage operation characteristics.

BACKGROUND ART

In this specification, reference is made to the following citedreferences identified with the reference numbers.

[Reference 1] “VLSI Memory Design” Kiyoo Itoh, p 162;

[Reference 2] Japanese Patent Laid-open No. Hei 2-24898 (correspondingU.S. Pat. No. 4,973,864);

[Reference 3] “Japanese Patent Laid-open No. Hei 10-3971 (correspondingU.S. Pat. No. 5,854,562);

[Reference 4] “1996 Symposium on VLSI Circuits Digests of TechnicalPapers, pp. 104-105;

FIG. 26.1 of the [Reference 1] is a sensing system circuit diagram ofthe standard DRAM (Dynamic Random Access Memory). This diagram is aso-called shared sense-amplifier structure (having the structure whereone sense amplifier line is used in common by the right and left memorymats). FIG. 18 shows a circuit diagram where this structure is omitted.A C100 and a M100 form a memory cell wherein the M100 indicates a chargetransfer NMOS transistor, while VPL indicates a plate voltage. BL[n],/BL[n] are bit lines, WL[m] is word line, and a memory cell is disposedat an adequate intersection to form a memory array MA100. M101, M102,M103 are NMOS transistors and VBM is a power source voltage equal to ahalf of the data line voltage VDL. These elements are precharge circuit101 of the so-called half VDD precharge system for precharging the bitline to the VBM potential by turning ON the M103 from the M101. M200,M201 are PMOS transistors, while M202, M203 are NMOS transistors,forming a CMOS latch type sense amplifier 201. Moreover, M109 and M110are NMOS transistors to form a Y switch 103 a to selectively connect thebit lines BL[n], /BL[n] to the global bit lines GBL[p], /GBL[p] byturning ON the M109 and M110

FIG. 19 shows waveforms for the read operation of this memory. Here, anarray voltage VDL is set to a voltage which is equal to the power sourcevoltage VDD assumed as 1.0V. Moreover, the power source voltage VBM isassumed as 0.5V which is equal to a half of such power source voltageand a setup voltage of the word line is assumed as 2.5V.

A precharge signal EQ is negated at the time T0 and a word line WL[m] isasserted at the time T1. Thereby, the MOS transistor M100 in the memorycell selected by such word line turns ON to share the chargesaccumulated in the capacitor C100 within the memory cell and a parasiticcapacitance added to the bit lines BL[n], /BL[n] in order to generate apotential difference Vs for reflecting information within the memorycell on the bit lines BL[n], /BL[n].

Since the sense amplifier activate signals CSP and CSN are respectivelydriven to 1.0V and 0V, the bit line potentials BL[n], /BL[n] areamplified up to 1.0V and 0V. In this figure, since a YS[k] is asserted,a Y switch is turned ON and the global bit lines GBL[p], /GBL[p] arealso amplified simultaneously when the bit lines BL[n], /BL[n] areamplified.

The signal /BL[n] which is given the slash sign “/” before BL[n] amongthe signals explained above depends on the generally used expressionmethod and this signal /BL[n] means a complementary signal of BL[n].Moreover, a bracket [ ] is also the generally used expression method andthe signal BL[n], for example, means the typical expression of signalsof bus structure consisting of one or more signal lines such asBL[0]BL[1], BL[2]. This expression method is used in this specification.

FIG. 20(A) shows a result of simulation of sensing rate (tSENSE) of thesensing system circuit of DRAM of FIG. 8 conducted by the inventors ofthe present invention. The sensing rate (tSENSE) is defined, as shown inFIG. 20(B), as the time required until a potential difference of the bitlines BL, /BL is amplified up to the 60% of the power source voltage VDDfrom activation of the sensing amplifier. Temperature is assumed as twokinds of temperatures of −40° C. and 125° C. in terms of the junctiontemperature Tj. This analysis by the inventors of the present inventionhas proved as follows.

(A1) The sense time (tSENSE) is remarkably delayed as the power sourcevoltage is lowered.

(A2) When the power source voltage is equal to about 1.2V or less, thesense time in the higher temperature is further than that in the lowertemperature. It is because a drive current of the sensing amplifier ismainly governed with a diffusion current, in place of a drift current,among the drain current of the MOS transistor. In general, the diffusioncurrent very sensitively changes for temperature and a threshold valueof MOS transistor. Therefore, when a sense amplifier is used in the areawhere the diffusion current governs the operation in place of the driftcurrent, a sensing time changes to a large extent for fluctuation ofmanufacturing process of LSI and fluctuation of operation environment.This event may grow up to a problem that an yield rate of an LSI circuitis lowered. As a result, cost of LSI using DRAM of the circuit of suchstructure rises.

Moreover, FIG. 20(C) shows dependence of a delay time of a CMOS inverteron a power source voltage as an example of a delay time characteristic(tDLAY) of an ordinary CMOS logic circuit. Temperature is assumed as twokinds of temperatures of −40° C. and 125° C. in terms of junctiontemperature Tj as in the case of FIG. 20(A). This analysis by theinventors of the present invention has proved as follows.

(B1) Deterioration in the operation rate when the power source voltageis lowered is remarkably smaller than that of the sensing system of theexisting DRAM shown in FIG. 18.

(B2) The temperature characteristic in the lower voltage condition isdifferent in the CMOS inverter and the sensing system of the existingDRAM shown in FIG. 18.

From this fact, it can be understood that the DRAM circuit including theexisting sensing system shown in FIG. 18 and the logic circuit havingthe delay characteristic shown in FIG. 20(C) are not matched with eachother through the low voltage characteristics thereof. Here, matching ofa plurality of circuits means that dependence of delay characteristic onthe power source voltage and temperature is similar. For example, whenthe power source voltage is set to a lower value, the operation rate ofall circuits is delayed in the similar degree and when the temperatureis lowered, the operation rate of all circuits is also delayed in thesimilar degree.

When the DRAM including the existing sensing system as shown in FIG. 18and a logic circuit which are not matched are disposed simultaneously onthe same LSI, the operation rate during a low voltage operation of thelogic LSI including such DRAM is governed with the characteristic thatoperation rate of the DRAM is rather low under the lower temperature.For example, the operation rate of the LSI as a whole is governed withthe racing. Moreover, when the logic LSI including such DRAM is used ina plurality of operation modes where the power source voltage and theoperating frequency vary, the operating frequency in the low voltageoperation mode is extremely delayed because the DRAM is included.

Therefore, it is an object of the present invention to provide a senseamplifier which stably operates even under the low voltage condition.

DISCLOSURE OF INVENTION

A typical structure of the present invention is as follows. Namely, asemiconductor device comprises a word line (WL), a first bit line pair(BL, /BL), a memory cell (MC) provided at an intersection of the wordline and the first bit line pair, a second bit line pair (LBL, /LBL),switch circuits (ISO_SW_T, ISO_SW_B) for coupling the first bit linepair and second bit line pair, a sense amplifier including a firstcircuit (PSA) connected to the first bit line pair and a second circuit(MSA) connected to the second bit line pair, a first precharge circuit(PC1) for precharging the first bit line pair to a first prechargepotential and a second precharge circuit (PC2) for precharging thesecond bit line pair to a second precharge potential, wherein the secondcircuit is a circuit for amplifying one of the first bit line pair andone of second bit line pair to a first potential (VSS) receiving astorage signal of the memory cell and the other pair to a secondpotential (VDL), the first precharge potential is a voltage (VBM)between the first potential and the second potential and the secondprecharge potential is equal to the second potential.

Moreover, according to the other aspect of the present invention, asemiconductor device comprises a word line (WL), a first bit line pair(BL, /BL), a memory cell (MC) provided at an intersection of the wordline and first bit line pair, a capacitor pair including a firstcapacitor (C250) having a first electrode connected to one of the firstbit line pair and a second electrode connected to one of the second bitline pair and a second capacitor (C251) having a third electrodeconnected to the other of the first bit line pair and a fourth electrodeconnected to the other of the second bit line pair, a switch circuitincluding a first switch (M206) for connecting one of the first bit linepair and one of the second bit line pair and a second switch (M207) forconnecting the other of the first bit line pair and the other of thesecond bit line pair, a sense amplifier (SA) connected to the second bitline pair, a first precharge circuit (PC1) for precharging the first bitline pair to a first precharge potential and a second precharge circuit(PC2) for precharging the second bit line pair to a second prechargepotential.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing an embodiment of a sensing system circuit ofthe present invention.

FIG. 2 is a diagram showing an embodiment of the read operation of FIG.1.

FIG. 3 is a diagram showing a DRAM macro including logics using thesensing system circuit of the present invention.

FIG. 4 is a diagram showing an embodiment of a system LSI using the DRAMincluding logics of the present invention.

FIG. 5 is a diagram showing the other embodiment of the sensing systemcircuit of the present invention.

FIG. 6 is a diagram showing the read operation of FIG. 5.

FIG. 7 is a diagram showing the other embodiment of the sensing systemcircuit of the present invention using capacitors.

FIG. 8 is a diagram showing an embodiment of the read operation of FIG.7.

FIG. 9 is a diagram showing the other embodiment of a sense amplifier ofthe present invention.

FIG. 10 is a diagram showing the other embodiment of the sensing systemcircuit of the present invention.

FIG. 11 is a diagram showing an embodiment of the read operation of FIG.10.

FIG. 12 is a diagram showing an embodiment of the DRAM macro using ashared sense amplifier system.

FIG. 13 is a diagram showing an embodiment when the sensing systemcircuit of FIG. 1 is changed to a shared sense amplifier system.

FIG. 14 is a diagram showing an embodiment when the sensing systemcircuit of FIG. 5 is changed to the shared sense amplifier system.

FIG. 15 is a diagram showing an embodiment when the sensing systemcircuit of FIG. 7 is changed to the shared sense amplifier system.

FIG. 16 is a diagram showing an embodiment when the sensing systemcircuit of FIG. 10 is changed to the shared sense amplifier system.

FIG. 17 is a diagram showing a control system of the DRAM of the presentinvention where a circuit for detecting the end of operation of thesense amplifier is loaded.

FIG. 18 is a diagram showing the existing sensing system circuit.

FIG. 19 is a diagram showing an example of discussion of the readoperation of FIG. 18 discussed by the inventors of the presentinvention.

FIG. 20 is a diagram showing the results of simulations by the inventorsof the present invention of the low voltage characteristics of thesensing system circuit of FIG. 18 and the low voltage characteristics ofthe CMOS inverter.

FIG. 21 is a diagram showing the results of simulations by the inventorsof the present invention of the low voltage characteristics when thesensing system circuit of FIG. 18 is operated in the VDD prechargesystem.

FIG. 22 is a diagram showing an embodiment when a memory array is formedusing the sensing system circuit of the present invention shown in FIG.1, FIG. 5, FIG. 7, FIG. 10, FIG. 13, FIG. 14, FIG. 15 and FIG. 16.

FIG. 23 is a diagram showing an embodiment of a re-write method of thepresent invention.

FIG. 24 is a diagram showing a circuit for realizing a re-write methodof the present invention different from FIG. 22.

FIG. 25 is a diagram showing a re-write operation using the embodimentof FIG. 24.

FIG. 26 is a diagram showing a re-write operation of the presentinvention using the embodiment of FIG. 24 different from the embodimentof FIG. 25.

FIG. 27 is a diagram showing the read operation of the present inventionusing the embodiment of FIG. 24.

FIG. 28 is a diagram showing the write operation of the presentinvention using the embodiment of FIG. 24.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiments of the present invention will be explained indetail with reference to the accompanying drawings. Circuit elementsforming each function block of the preferred embodiments are formed,although not particularly limited, on only one semiconductor substratemade of single crystal silicon or the like with the well knowntechnology to form CMOSs (complementary MOS transistors). The P-type MOStransistor (MOSFET) can be discriminated by giving a sign ◯ to the gatethereof from an N-type MOS transistor (MOSFET).

Embodiment 1

FIG. 1 shows an embodiment of a typical sensing system circuit of thepresent invention. C100 and M100 form a memory cell (MC). The C100 is acapacitor for storing information within the memory cell, M100 is acharge transfer NMOS transistor and VPL is a plate voltage. BL[n] and/BL[n] are bit lines, WL[m] is a word line and a memory cell is disposedat the adequate intersection to form a memory array 100. Here, theembodiment based on the folded bit line structure is shown but a openbit line structure may also be introduced. In this figure, M107 and M108are NMOS transistors to form a Y switch Y-SW, while the local bit linesLBL[n], /LBL[n] can be selectively connected to the global bit linesGBL[P], /GBL[p] by turning ON the M107 and M108.

A sense amplifier SA1 employed in this invention has the followingcharacteristics. Namely, the sense amplifier SA1 includes a pre-senseamplifier PSA connected to the bit line pair BL[n], /BL[n] (these canalso be abbreviated as “BL”) and local bit line pair LBL[n], /LBL[n](these can also be abbreviated as “LBL”). Moreover, switch circuits(ISO_SW_T, ISO_SW_B) for controlling connection and isolation of the BLand LBL are also provided. The PSA includes a N-type MOSFET pair (M204and M205) with the gate thereof connected to the BL and the sourcethereof connected in common and this N-type MOSFET pair operates as thedifferential MOSFET pair for receiving a signal at the gate. Moreover, amain sense amplifier MSA is a circuit including the CMOS latch typesense amplifier as the basic structure. In the MSA, the P-type MOSFETpair M200 and M201 assures that the gate and drain thereof arecross-connected and the sources are connected in common. Moreover, theN-type MOSFET pair M202 and M203 assures that the gate and drain thereofare cross-connected and the sources are connected to the drains of theN-type MOSFET pair of the PSA.

FIG. 9 of the [Reference 2] shows a sense amplifier including the PSAand MSA explained above when attention is paid to the format of circuit.Moreover, FIG. 16 of the [Reference 2] shows a circuit operation.However, the sense amplifier of the [Reference 2] relates to the SRAM.Namely, this reference does not suggest application to the DRAM of thepresent invention and therefore any consideration is never taken for theswitch circuits (ISO_SW_T. ISO_SW_B) explained below.

The second characteristic of the present invention is that the switchcircuits (ISO_SW_T. ISO_SW_B) for controlling connection and isolationof the BL and LBL are provided corresponding to difference of prechargepotentials of the BL and LBL. M206 and M207 are NMOS transistors.Re-write operation can be realized by electrically connecting the BL andLBL with this switch circuit and then transferring the data amplifiedwith the MSA to the BL from the LBL.

The third characteristic of the present invention is that the BL isprecharged to VDL/2, while the LBL to VDL. The M101, M102, M103 are NMOStransistors and VBM is a power source voltage equal to a half voltage ofthe data line voltage VDL. These elements form a so-called prechargecircuit 101 of the half VDD precharge system for precharging the bitlines BL[n], /BL[n] (first bit line pair) to the VBM potential (firstprecharge voltage) by turning ON the M103 from the M101. On the otherhand, M104, M105 and M106 are transistors and these elements form aso-called precharge circuit 102 of the VDD precharge system forprecharging the LBL (second bit line pair) to the VDL potential (secondprecharge voltage) by turning ON these MOS transistors.

FIG. 2 shows an example of the read operation waveforms of the memory ofFIG. 1. Here, an array voltage VDL is set to a voltage which is equal tothe power source voltage VDD of chip and is assumed as 1.0V forsimplified explanation. Moreover, the VBM is assumed as a voltage of0.5V which is equal to a half of the VDL, while the setup voltage of theword line is assumed as 2.5V.

The precharge signals EQ_BL and EQ_LBL are negated at the time T0 andthe word line WL[m] is asserted at the time T1. Therefore, the transferMOS transistor M100 in the memory cell selected with such word lineturns ON, sharing the charges accumulated within the capacitor C100 inthe memory cell and the parasitic capacitance added to the bit linesBL[n], /BL[n] and thereby a potential difference Vs reflectinginformation within the memory cell is generated between the bit linesBL[n] and /BL[n].

A sense amplifier is activated at the time T2 by driving a senseamplifier activate signal CSN to 0V, potential differences of the bitlines BL[n] and /BL[n] are amplified up to 1.0V and 0V and thesepotential differences are outputted to the local bit lines LBL[n] and/LBL[n]. Since YS[k] is asserted in this figure, the Y switch is turnedON and the global bit lines GBL[p], /GBL[p] are also amplifiedsimultaneously with amplification of the bit lines BL[n], /BL[n].

Moreover, a wrote back signal RBK is asserted at the time T2′ to executethe re-write to the memory cell by transferring the amplified signals ofthe local bit lines LBL[n], /LBL[n] to the bit lines BL[n], /BL[n].

The write back signal RBK and word line WL[m] are negated at the timeT3, the precharge signals EQ_BL and EQ_LBL are asserted at the time T4,thereby the bit lines BL[n] and /BL[n] are precharged to 0.5V, while thelocal bit lines LBL[n], /LBL[n] are precharged to 1.0V.

FIG. 3 shows an embodiment of the DRAM macro using the sensing systemcircuit of FIG. 1. Numeral 500 designates a DRAM macro. Numeral 501designates an indirect peripheral circuit consisting of a commanddecoder 502, a read/write amplifier 503 and a power source circuit 504.Moreover, BA0 to BA7 are memory banks. Each bank is composed of a timingcontrol circuit TG, a column selection circuit Y-DEC, a row decoderX-DEC and a plurality of sense amplifiers 506 a, 506 b. The sensingsystem circuit shown in FIG. 1 corresponds to 506 a or 506 b of FIG. 3and is disposed in each bank in the manner that the two units areprovided opposed with each other. A control signal of the word lineWL[m] or the like of FIG. 1 is controlled with the row decoder, timingcontrol circuit and column selection circuit or the like. The GBL0,/GBL0 form a pair of the global bit lines and are laid in parallel tothe bit lines BL0, /BL0 and the eight sense amplifiers of the sensingsystem circuit indicated with 506 a, 506 b of each bank are connected toa pair of global bit lines GBL (it means that a degeneration degree is8). The GBL is provided crossing the memory bank and is connected to ablock 503 including a read/write amplifier RW-AMP provided correspondingto the memory bank. The read/write amplifier RW-AMP is connected, via aselected as required or in direct, to the external input/output datasignal line DQ. The control signal CNT and address signal ADD of theDRAM macro are inputted to a command decoder C-DEC and this C-DEC sendsa control signal to TG or the like to execute the predetermined read orwrite operation.

In the embodiment of FIG. 3, it is a characteristic that since anindependent sensing system circuit is formed for each bank and moreovera timing control circuit 507 is provided within each bank, each bank canbe operated independently with the control from the command decoder 502.The throughput of the DRAM macro can be enhanced with the so-calledinterleave system by the independent operation of each bank.

FIG. 4 shows the entire part of a logic LSI (400) including the DRAMcomprising the DRAM macro 500 shown in FIG. 3. VDD and VSS indicate thecore power source and its ground, while VDDQ and VSSQ indicate the I/Opower source and its ground. For example, the core power source voltageis 1.0V and the I/O power source voltage is 3.3V. OUT0 to OUTx indicatethe output signals, while IN0 to INy are input signals and I/00 to I/0 zare input/output signals, respectively. Moreover, 401 designates an I/Ocircuit for interface of an internal signal of the chip and an externalsignal of the chip. Numeral 402 designates a logic circuit formed of aninverter, NAND gate or the like and 403 designates the DRAM macro shownin FIG. 3. As an example of 402, a microprocessor (CPU) or DSP or SRAMor the like may be listed although not particularly restricted

FIG. 21 shows a result of simulation conducted for evaluating thecharacteristics of the sensing system circuit of the present inventionshown in FIG. 1. This simulation result means a result of calculationwhen the bit lines BL[n], /BL[n] are precharged to the VDD in thesensing circuit of the DRAM shown in FIG. 18. A circuit configuration issame as that of FIG. 20(A) except for the precharge system. Thesimulation result is also the same as that of FIG. 19, except for thedrive method of the sense amplifier activate signal that the CSP isfixed to the VDD potential and the CSN is driven to the VSS potentialfrom the VDD potential. The inventors of the present invention hasproved as follows from the analysis explained above.

(C1) The sense time (tSENSE) is delayed as the power source voltage islowered but a degree of such delay is rather gradual in comparison withFIG. 20(A) and is well matched with the characteristics of the CMOSinverter (FIG. 20(C)).

(C2) In the range where at least the power source voltage is equal to orhigher than 0.8V, the sense time is rather fast than that when thetemperature is lower. It is because a drive current of the senseamplifier is mainly governed with a drift current, in place of adiffusion current, among the drain current of the MOS transistor and itis well matched with the characteristics of the CMOS inverter (FIG.20(C)).

As explained above, the low voltage operation characteristic of thesensing system circuit of the VDD precharge system is more excellentthan that of the half VDD precharge system to a remarkable extent and itcan be understood that the DRAM sense circuit of the VDD prechargesystem is well matched with the CMOS inverter. Only a result of the VDDprecharge system is indicated here to simplify the explanation, but theVDD precharge is executed before the drive of the sense amplifier inregard to the amplification of the local bit lines even in the sensingsystem circuit of the present invention shown in FIG. 1. Namely, theseprecharge systems are essentially identical and the characteristicsshown in FIG. 21 and explained above can also be obtained.

Moreover, the VDD precharge system has a problem that a particular cellsuch as a dummy cell or the like is required for generation of thereference voltage. However, in the present invention, such dummy cellfor the reference voltage has been eliminated by isolating the bit linesBL[n], /BL[n] connected to the memory cell and the local bit linesLBL[n], /LBL[n] for the DC element, using the half VDD precharge systemfor the bit lines BL[n], /BL[n] and using the VDD precharge system forthe local bit lines LBL[n], /LBL[n].

As explained above, the sensing system circuit of the present inventionshown in FIG. 1 is characterized as follows.

(D1) A sense time in the lower temperature condition is faster than thatin the higher temperature condition even when a voltage is low.

(D2) Deterioration in a sense rate under the low voltage condition iscontrolled to the same degree as the deterioration of delay time of theCMOS inverter shown in FIG. 20(B).

The characteristic of the item (D1) is attained because a drive currentof the sense amplifier in the present invention is mainly governed witha drift current, in place of a diffusion current, among the draincurrent of MOS transistor. In general, change of a diffusion current isvery sensible to temperature and threshold value of MOS transistor.Therefore, when a sense amplifier is used in the area where is mainlygoverned with a diffusion current in place of a drift current like thesensing system circuit shown in FIG. 18, a sense time changes largelyfor fluctuation in manufacturing processes of LSI and fluctuation inoperation environment of LSI. This change is developed up to a problemthat an yield rate of the LSI circuit is lowered. As a result, amanufacturing cost of LSI using the DRAM of the circuit configurationexplained above rises. Accordingly, it can be said that the sensingsystem circuit of the present invention is resistive to fluctuation inmanufacturing processes of LSI and fluctuation in operation environmentof LSI. Moreover, the sensing system circuit has a circuit configurationhaving a higher yield rate.

Moreover, with the characteristics of the items (D1), (D2), the DRAMmacro 403 has the characteristic which is well matched with the lowvoltage characteristic of the logic circuit 402 of FIG. 2. Therefore, itcan no longer be said that any one of above elements will mainly controlthe low voltage characteristic and the DRAM macro 402 can be includedinto the logic LSI without deterioration of the final LSI characteristicto a large extent.

In addition, the sensing system circuit of the present invention of FIG.1 has a characteristic that this circuit does not require a particularcell such as a dummy cell which has been required for the existing VDDprecharge system, while having the characteristic of the existing VDDprecharge system explained in the items (D1), (D2). Thereby, themanufacturing process and circuit can be simplified remarkably and theyield rate can also be improved to realize the effect for lowmanufacturing cost of LSI.

The symbol MOS in FIG. 1 of which gate electrode is indicated with awhite box such as M206 indicates a high dielectric voltage MOStransistor formed of a thick gate oxide film, while that of which gateelectrode is indicated with a line like M202 indicates a MOS transistorformed of a thin gate oxide film. A method for using the MOS of twokinds of the gate oxide film thickness has a merit, although notparticularly restricted, that an adequate voltage can be impressed tothe gate electrode through the structure explained in regard to thisembodiment. Since it is enough when the dielectric strength of oxidefilm in the thin oxide film MOS explained previously is basicallycapable of covering the power source voltage VDD, a high speed MOStransistor may be used. The thick oxide film MOS explained later iscapable of using the MOS which is the same as that used in the outputstage of the I/O circuit of LSI and it is also enough when thedielectric strength of the oxide film can basically cover the voltage upto the I/O voltage VDDQ. An example of the basic selective use of theMOS transistor as in the case of FIG. 1 will be shown in the subsequentdrawings. Moreover, there is no particular restriction in the thresholdvoltage of MOS transistors. A structure of the DRAM macro using thesensing circuit of the present invention and the logic LSI includingDRAM using the same macro is not particularly restricted to thestructure shown in FIG. 3 and FIG. 4.

Moreover, in above embodiment, it is explained that the potentialamplitude of the bit line appears as VSS (0V) and VDL (1V). But thismerit is particularly distinctive when VDL is equal to or less than 1.8Vand moreover is ranged from 1.8V to 0.5V. This merit is common in thefollowing embodiments.

Embodiment 2

FIG. 5 shows another embodiment of the sensing system circuit of DRAM ofthe present invention. In FIG. 1, the MOS transistors M204, M205 in thesense amplifier connected with the bit lines BL[n], LBL[n] arerespectively connected in series to M202 and M203. Meanwhile, in thesense amplifier SA2 of FIG. 5, M208 and M209 corresponding to M204 andM205 are connected in parallel to M202 and M203 to form a presenseamplifier PSA in combination with M208 and M209. Moreover, the mainsense amplifier MSA part is formed as a latch type circuit in which theM200 to M203 are included, sources of the M202 and M203 are coupled incommon and the CMOS inverter is cross-connected. MSA and PSA arerespectively isolated with the drive lines CSN and PRECSN forindependent control.

A sense amplifier which is similar only in the circuit format is shownin FIG. 1 of the [Reference 3]. However, in the circuit of the[Reference 3], it is not considered, unlike the present invention, thatthe precharge level is different in the bit line BL and local bit lineLBL and the switch circuits (M206, M207) are used for isolation andcoupling of the BL and LBL.

FIG. 6 shows an example of the read operation waveforms of the sensingsystem of the embodiment shown in FIG. 5. The same explanation iseliminated by explaining only the part which is different from the readoperation shown in FIG. 2. At the time T1, the word line WL[m] isasserted and simultaneously the drive signal PRECSN (source voltage ofM208 and M209) of the pre-sense amplifier 202 b shown in FIG. 5 isdriven to −0.5V. Thereby, since the bit lines BL[n] and /BL[n] areconnected to the gate electrodes of M208 and M209, the local bit linesLBL[n], /LBL[n] precharged to 1.0V are discharged as shown in the figuredepending on the potentials of the bit lines BL[n], /BL[n]. At the timeT2, the main sense amplifier 202 a is activated by driving the CSN to 0Vto amplify the potential difference of the local bit lines LBL[n],/LBL[n] generated through the discharge operation.

In the system of the embodiment shown in FIG. 1, the M204 and M205 are apart of the drive MOS transistor of the local bit lines LBL[n], /LBL[n]but since only a voltage around 0.5V is impressed to M204 and M205 evenafter the activation of the sense amplifier, a drive force of the localbit lines LBL[n], /LBL[n] is controlled with a weak drive force of theM204 and M205. Therefore, the MOS transistor having a low thresholdvalue must be used for the M204 and M205 for the operation under a lowervoltage condition in view of obtaining a large drive force even if onlya voltage equal to a half of the power source voltage is impressed tothe gate electrode. On the other hand, in the embodiment of FIG. 5, onlythe MOS transistors M202 and M203 drive the local bit lines LBL[n],/LBL[n] when the sense amplifier is activated and the MOS transistorsM208 and M209 are used in the pre-sense period (the period from the timeT1 to the time T2 in FIG. 6). Therefore, even when the low thresholdvoltage MOS transistors are not used for M208 and M209, high speedoperation of the main amplifier 202 a can be realized.

In the embodiment of FIG. 6, a drive signal PRECSN of the pre-senseamplifier 202 b is driven up to −0.5V to drive the pre-sense amplifierconsisting of M208 and M209 but the drive voltage of PRECSN is notparticularly restricted. However, since only a voltage of about 0.5V isapplied at the time T1 to the gate electrodes of M208 and M209, the M208and M209 can drive the local bit lines LBL[n], /LBL[n] at a high speedwhen the PRECSN is driven up to a negative voltage. Moreover, when thePRECSN is driven up to a negative voltage, a voltage difference betweenthe source and gate of the M208 and M209 becomes larger. Accordingly,the local bit lines LBL[n], /LBL[n] can be driven with a drain currentresulting from the drift current to match the pre-sense timecharacteristic up to the time T2 from the time T1 and the delaycharacteristic of the logic circuit.

When the PRECSN is driven up to a negative voltage, a drive force of theM208 and M209 becomes too large and when a voltage difference of about100 mV which is enough to drive the main sense amplifier 202 a isgenerated on the local bit lines LBL[n], /LBL[n], and therefore it isprobable that both local bit lines LBL[n], /LBL[n] may be driven to thepotential near to about 0.5V. Under this condition, the effect attainedby precharging the local bit lines LBL[n], /LBL[n] connected to the mainsense amplifier to the VDD is lost. In order to prevent such event whilethe PRECSN is driven to a negative voltage, the gate length Lg of theM208 and M209 is increased or the gate width W is narrowed to adjust acurrent of M208 and M209 to drive the local bit lines LBL[n], /LBL[n].

Embodiment 3

FIG. 7 shows another embodiment of the sense amplifier of the presentinvention. In this embodiment, unlike FIG. 1 and FIG. 5, capacitors C250and C251 formed of MOS transistors are connected between the bit linesBL[n], /BL[n] and the local bit lines LBL[n], /LBL[n]. In theembodiments of FIG. 1 and FIG. 5, a voltage difference of the bit linesBL[n], /BL[n] connected to the memory cell is detected as a draincurrent difference flowing corresponding to the gate voltage thereof byconnecting the bit lines BL[n], /BL[n] to the gate electrode of the MOStransistor of pre-sense amplifier PSA in the sense amplifier. Meanwhile,in this embodiment, a voltage difference of the bit lines BL[n], /BL[n]connected to the memory cell is transferred to the local bit linesLBL[n], /LBL[n] through the capacitance couple (so-called AC coupling)of the capacitors C250 and C251.

FIG. 8 shows an example of the read operation waveforms of the sensingsystem in the embodiment of FIG. 7. Here, only a part different from theread operation explained with reference to FIG. 2 and FIG. 6 will beexplained to eliminate duplicated explanation. When the word line WL[m]is asserted at the time T1, a voltage difference Vs1 corresponding tothe information within the memory cell is generated to the bit linesBL[n], /BL[n] connected to the memory cell. This potential difference istransferred to the local bit lines LBL[n], /LBL[n] through thecapacitance coupling of the capacitors C250, C251 of FIG. 7 and therebya potential difference Vs2 is generated in the local bit lines LBL[n],/LBL[n]. Thereafter, the sense amplifier activate signal CSN is assertedat the time T2 to activate the sense amplifier and the potentialdifference Vs2.

Here, although it is not particularly restricted, it is preferable thata structure of the capacitors C250 and C251 is formed with MOS capacitorbased on the NMOS transistor. The capacitor utilizing a gate capacitanceof the MOS transistor has a property that capacitance changes dependingon the potential difference between the gate and source/drain. Namely,when the potential difference between the gate and source/drain islarge, a channel is formed to the MOS transistor and a capacitance islarge and when the potential difference between the gate andsource/drain is small, a channel disappears the a capacitance becomessmall. Hereinafter, such effect is called the capacitance modulationeffect.

In FIG. 8, a potential difference Vs2 of the local bit lines LBL[n],/LBL[n] is amplified by activating the sense amplifier at the time T2,but the capacitance of the bit lines BL[n], /BL[n] becomes large via thecapacitance coupling through the C250 and C251 from the local bit linesLBL[n], /LBL[n]. Therefore, the following points must be taken intoconsideration to drive, at a high speed, the local bit lines LBL[n],/LBL[n] with the sense amplifier.

(E1) Any one of local bit lines LBL[n], /LBN[n] driven for the lowvoltage side (LBL[n] in FIG. 8) must be driven at a high speed to therow side, when it is driven, by virtually lowering a parasiticcapacitance added to the bit line /BL[n]. For this purpose, it ispreferable to provide a capacitor C251 having a smaller capacitancebetween the local bit line /LBL[n] and the corresponding bit line/BL[n].

(E2) Any one of local bit lines LBL[n], /LBL[n] driven for the highvoltage side (LBL[n] in FIG. 8) must preferably be held in the highvoltage side, when it is driven, without driving the local bit line/LBL[n] to the low voltage side with the parasitic capacitance when thesense amplifier is driven by virtually increasing the parasiticcapacitance added to the bit line BL[n]. For this purpose, it ispreferable that a capacitance C250 provided between the local bit lineLBL[n] and the corresponding bit line BL[n] has a large capacitance.

The contents of items (E1) and (E2) can be realized automatically withthe capacitance modulation effect explained above by using a capacitorutilizing the NMOS transistor as the capacitors C250 and C251.

A connection method when the MOS transistor is used for the capacitorsC250 and C251 (the gate electrode is connected to the local bit line inFIG. 7) and the method of setting the substrate potential are notparticularly restricted. However, the relationship between Vs1 and Vs2of FIG. 8 is determined based on the charge sharing between thecapacitance Ca of the capacitors C250, C251 and the parasiticcapacitance Cp added to the local bit lines LBL[n], /LBL[n]. Namely,Vs2=Vs1*Ca/(CP+Ca). Therefore, it is recommended to make small Cp asmuch as possible when Ca is assumed to be constant. A value of Cp can bereduced as much as a junction capacitance of a diffusion layer of theMOS transistor forming the C250 and C251 by connecting the gateelectrode to the local bit line such as C250 and C251.

Embodiment 4

An embodiment of the sensing system circuit of the present invention isshown in FIG. 1, FIG. 5 and FIG. 7 but it is enough, in short, when thebit lines BL[n], /BL[n] connected to the memory cell and the local bitlines LBL[n], /LBL[n] connected to the sense amplifier are electricallyisolated, the bit lines BL[n], /BL[n] are precharged to the half VDD,the local bit lines LBL[n], /LBL[n] are precharged to the VDD and avoltage difference is generated in the local bit lines LBL[n], /LBL[n]corresponding to the voltage difference of the bit lines BL[n], /BL[n]generated when the word line WL[m] is asserted at the time of readoperation. A structure of the sense amplifier connected between the bitlines BL[n], /BL[n] and the local bit lines LBL[n], /LBL[n] is notrestricted to that shown in FIG. 1, FIG. 5 and FIG. 7. For example, thestructure shown in FIG. 9 may be used.

FIG. 9 shows a structure in which the CMOS latch type sub-senseamplifier SSA consisting of the MOS transistors of M290 to M293 is addedto the embodiment shown in FIG. 5. The main sense amplifier MSA includesM200 to M203 and is same as the MSA of FIG. 5. But, in the pre-senseamplifier PSA (M208, M209), the sources which are connected in commonare connected to the input/output node of the sub-sense amplifier SSA.The activate signals CSP2 and CSN2 of the sub-sense amplifier areprecharged, before the activation, to the VBM potential as shown by thewaveforms of FIG. 9 and are driven to 1.0V and 0V at the time T2 in thesame timing as the sense amplifier activate signal CSN.

When the sub-sense amplifier 290 is activated, the bit lines BL[n],/BL[n] which are precharged to the half VDD are amplified andsimultaneously a current flowing into the M208 and M209 accelerates theamplifying operation of the main sense amplifier 202 a consisting ofM200 to M203. As a result, the local bit lines LBL[n], /LBL[n]precharged to the VDD are amplified at a high speed up to 1.0V and 0V.Moreover, since the sub-sense amplifier simultaneously amplifies the bitlines BL[n], /BL[n], the charging time of the bit lines BL[n], /BL[n]can be shortened when the write back signal RBK is activated at the timeT2′.

When it is not so much required to consider the rate of re-writeoperation, the re-write operation may be conducted only with thesub-sense amplifier 290 by eliminating the M206 and M207.

The effect of shortening the time required for the re-write operationcan also be obtained by adding a sub-sense amplifier 290 consisting ofM290 and M293 to the bit lines BL[n], /BL[n] of the sensing systemcircuit of the present invention of FIG. 1, FIG. 5 and FIG. 7. Moreover,it is of course possible to delete the NMOS transistors M206 and M207for re-write operation.

Various structures of sense amplifier can be considered when there is noparticular restriction on the number of transistors and on the area asexplained above and these structures are also not placed under theparticular restriction.

Embodiment 5

As the other embodiment, it is also possible that the bit lines BL[n],/BL[n] connected to the memory cell and is precharged to the half VDDand the local bit lines LBL[n], /LBL[n] connected to the sense amplifierare electrically isolated immediately before the sense amplifier isactivated, the local bit lines LBL[n], /LBL[n] are simultaneously driventhrough the capacitance coupling and the local bit lines LBL[n], /LBL[n]are precharged to the VDD when the sense amplifier is activated. FIG. 1shows an embodiment to realize this purpose.

The sensing system circuit of the present invention of FIG. 10 inserts,in comparison with the sensing system circuit of FIG. 18, the PMOStransistors M260 and M261 to the bit lines BL[n], /BL[n] of FIG. 18 tocontrol these transistors with a bit line isolation signal /SH.

FIG. 11 shows an example of read operation waveforms of the sensingsystem in the embodiment of FIG. 10. In this figure, only a partdifferent from the read operation of FIG. 19 is explained to eliminateduplicated explanation. After the word line WL[m] is asserted at thetime T1, the bit line isolation signal /SH is driven up to 2.5V from−0.8V at the time T1′. Thereby, the bit lines BL[n], /BL[n] areelectrically isolated from the local bit lines LBL[n], /LBL[n] andmoreover the local bit lines LBL[n], /LBL[n] are simultaneously drivento the high level through the capacitance coupling between the gate anddrain of the M260 and M261 or between the capacitances of gate andsource. Thereafter, the sense amplifier 201 is driven at the time T2 toamplify the memory cell information to the local bit lines LBL[n],/LBL[n]. At the time T2′, the bit line isolation signal /SH is driven upto −0.8V from 2.5V, the bit lines BL[n], /BL[n] and the local bit linesLBL[n], /LBL[n] are electrically connected and the bit lines BL[n],/BL[n] are driven to 1V and 0V to conduct re-write operation to thememory cell.

When the sense amplifier is driven at the time T2, the local bit linesLBL[n], /LBL[n] connected to the sense amplifier are driven up to avoltage near the power source voltage from the voltage near to 0.5V.Therefore, the low voltage characteristic which is similar to that whenthe sensing system circuit of FIG. 18 is precharged to the VDD can beattained.

In FIG. 10, the PMOS transistor has been used for the M260 and M261, butthe NMOS transistor may also be used. In this case, the signal /SH isdriven to a negative voltage from a positive voltage at the time T1′ andthe local bit lines LBL[n], /LBL[n] are simultaneously driven to the lowvoltage side through the capacitance coupling. As a result, thecharacteristic similar to that when the sensing system circuit of FIG.18 is precharged to the voltage VSS can be obtained. In general, sincethe VDD precharge system is mainly used for driving of the bit lines bythe NMOS transistors at the time of driving the bit lines with the senseamplifier, this VDD precharge system has the more excellent low voltagecharacteristic than the VSS precharge system. However, the low voltagecharacteristic which is remarkably preferable to that of the half VDDprecharge system can be attained with the VSS precharge system.

The sensing system circuit disclosed in the [Reference 4] may beconsidered as a technique similar to the embodiment of the presentinvention shown in FIG. 10. The [Reference 4] discloses that the bitlines connected to the memory cell are isolated electrically from thesense amplifier before activation of the sense amplifier (senseoperation 1), thereafter the bit lines are driven to the high levelthrough the capacitance coupling with the capacitor adding the bit linesin the side where the sense amplifier is connected after a constantperiod (sense operation 2) and thereafter the bit lines activate thesense amplifier (sense operation 3).

This embodiment of the present invention is different from the[Reference 4] mainly in the following two points.

(F1) In the method of the [Reference 4], it is necessary to add thecapacitance for driving the bit lines in the side connected to the senseamplifier through the capacitance coupling. In the method of the presentinvention, the signal /SH is set to a sufficiently large value andmoreover the local bit lines are driven with the parasitic capacitancesof the M260 and M261. Therefore, it is not particularly required to addthis capacitance.

(F2) In the method of the [Reference 4], the timing up to the senseoperation 3 from the operation 1 is required as explained above up tothe activation of the sense amplifier. In the present invention, thesense operation 1 and sense operation 2 can be done simultaneously.

In order to realize close capacitance coupling between the signal /SHwhen this signal is activated and the local bit lines LBL[n], /LBL[n],it is also possible to add respectively the capacitances between thegate electrode of M260 and the local bit line LBL[n] and between thegate electrode of M260 and local bit line /LBL[n]. In this case, thecapacitor may be formed of the NMOS transistor. Moreover, it is alsorequired to add the capacitor as in the case of the [Reference 4], butthe method of the present invention is only a complementary methodhaving a merit that a capacitor having a small capacitance value mayalso be used. Moreover, the merit that the sense operation 1 and thesense operation 2 can be conducted simultaneously which has beenrequired for the [Reference 4] is still maintained.

Embodiment 6

The sensing system circuit explained in above embodiments is illustratedas a circuit of the format not introducing the so-called shared senseamplifier system but this sensing system circuit is never restrictedthereto. FIG. 12 shows an embodiment where the shared sense amplifiersystem is used. Here, a hierarchical word line drive system, which hasnot been particularly restricted in the embodiments shown in the figuresup to FIG. 11, is used. SWD611 is a sub-word decoder, Y-DEC605 is a Ydecoder, X-DEC&MWD608 indicates an X-decoder and main word driver. BL0,/BL0 and BL1, /BL1 respectively indicate the bit line pairs which areconnected to one sensing system circuit 606 a. The global bit linesGBL0, /GBL0 are wired in the direction (direction parallel to the wordline) crossing in orthogonal the bit lines. The control signal of theDRAM circuit 600 and data line are omitted.

Since many components of the sensing system circuit may be used incommon with two pairs of bit lines by introducing the shared senseamplifier system, a memory cell occupation coefficient may be enhanced.When the sensing system circuit of the present invention is used for theDRAM of higher integration density which is used for the main memory orthe like of the microprocessor called so-called the general purposeDRAM, instead of the DRAM macro included into the logic LSI, it isimportant to enhance the memory cell occupation coefficient. In suchapplication field, it is enough when the sensing system circuit of thepresent invention is used in the shared sense amplifier system. Anembodiment where the sensing system circuit of FIG. 1, FIG. 5, FIG. 7and FIG. 10 is changed to the shared sense amplifier system will beexplained below.

FIG. 13 shows an embodiment where the sensing system circuit of FIG. 1is changed to the shared sense amplifier system and the memory array MAis omitted. In the shared sense system, the right and left memory mats(vertical memory mats in FIG. 13) are required but the main senseamplifier including M200 to M203 is used in common by the right and leftmats. Meanwhile, the pre-sense amplifier is provided with a firstpre-sense amplifier PSA_UP including M204 and M205 for the first mat anda second pre-sense amplifier PSA_DN including M232 and M233 for thesecond mat. Moreover, the precharge circuits (PC1 a, PC1 b) forVBM(VDL/2) are provided respectively for the right and left mats.

In the circuit of FIG. 13, the NMOS transistors up to M233 from M230 andthe half VDD precharge circuit 101 b consisting of M101 b to M103 b areadded to FIG. 1 and the memory cell is connected to the bit linesBL_UP[n], /BL_UP [n] and BL_DN[n], /BL_DN[n]. Explanation of the readoperation of the embodiment of FIG. 13 is omitted here because it may beassumed easily from the embodiments of FIG. 1 and FIG. 2. Here, it isimpossible to simultaneously read or write the data from or to thememory cell connected to the bit lines BL_UP[n], /BL_UP[n] or the memorycell connected to the bit lines BL_DN[n], /BL_DN[n] but it is possibleto access to any one of them with the common sense amplifier.

FIG. 14 shows an embodiment where the sensing system circuit of FIG. 5is changed to the shared sense amplifier system. In the embodiment ofFIG. 14, the shared sense amplifier system is introduced and the bitlines are formed in the hierarchical structure. SUBA_UP−1 to SUB_UP−jare sub-memory arrays, each of which is composed of the pre-senseamplifier PSA1(203 b) including the sub-bit lines BL[n]−1, /BL[n]−1 andM222 and M223 and the half VDD precharge circuit 101. SUBA_DN−1 toSUBA_DN−j are also sub-memory arrays which are disposed, as the physicallayout thereof, in the opposite side of the SUBA_UP−1 to SUBA_UP−j inthe sense amplifier 203 a, VDD precharge circuit 102 and Y switch 103.The main sense amplifier MSA2(203 a) and VDL precharge circuit PC2 areprovided in common to a plurality of sub-memory arrays. The readoperation of the embodiment of FIG. 14 can be easily estimated from theoperations of FIG. 5 and FIG. 6 and therefore explanation of such readoperations is omitted here.

In the sensing system circuit of DRAM, it has been required to increasea capacitance of the capacitor C100 within a memory cell so that apotential difference Vs of the bit lines read to the bit lines from thememory cell after the assertion of the word lines has a certain valueduring the low voltage operation. Therefore, here rises a problem thatdifficulty of process becomes high. Since the bit lines are formed inthe hierarchical structure in the embodiment of the present invention ofFIG. 14, the bit lines BL[n]−1, /BL[n]−1 can be shortened and the numberof memory cells connected to these bit lines can be reduced. Therefore,a capacitance value of the capacitor C100 in the memory cell can bereduced and thereby this problem during the low voltage operation can besolved.

FIG. 15 shows an embodiment where the sensing system circuit of FIG. 7is changed to the shared sense amplifier system. Here, memory cells areomitted. In comparison with the embodiment of FIG. 7, M300 a and M301 aare provided for electrically isolating the local bit lines and the halfVDD precharge circuit 101 b consisting of NMOS transistors M206 b, M207b, M300 b, M301 b and M101 b, M102 b, M103 b is added.

The read operation in the embodiment of FIG. 15 can be easily estimatedfrom the embodiments of FIG. 7 and FIG. 8 and therefore explanation ofthis read operation is omitted here. Here, it is impossible tosimultaneously conduct the read or write operation to or from the memorycell connected to the bit lines BL_UP [n], /BL_UP[n] or to the memorycell connected to the bit lines BL_DN[n], /BL_DN[n], but any one of thememory cells can be accessed by driving any one of the SH_UP or SH_DN tothe voltage of about 2.5V.

FIG. 16 shows an embodiment where the sensing system circuit of FIG. 10is changed to the shared sense amplifier system. Here, the memory cellsare omitted. In comparison with the embodiment of FIG. 10, the M262 andM203 are newly provided and are then controlled with a bit lineisolation signal /SH_DN.

The read operation of the embodiment of FIG. 16 can easily be estimatedfrom the embodiments of FIG. 10 and FIG. 11 and therefore explanation ofthis read operation is omitted here. It is impossible to simultaneouslyconduct the read or write operation from or to the memory cell connectedto the bit lines BL_UP[n], /BL_UP[n] or to the memory cell connected tothe bit lines BL_DN[n], /BL_DN[n], but any one of memory cells can beaccessed by driving any one of the /SH_UP or /SH_DN to the voltage ofabout 2.5V.

In the following embodiments, the sensing system circuit is shown, tosimplify the explanation, as the circuit format not introducing theso-called shared sense amplifier system, but it is apparent the circuitmay be changed to the shared sense amplifier system as explained above.

Embodiment 7

One of the characteristics of the sensing system circuit of the presentinvention explained above is that the local bit lines LBL[n], /LBL[n]connected to the sense amplifier are precharged to the voltage VDD Owingto the VDD precharge, the low voltage characteristic in the sense timeof the sense amplifier is matched with that of the logic circuit but thesensing system circuit of the present invention also has the othermerits. One of such merits is that the end of amplification by the senseamplifier can be detected easily. FIG. 17 shows an embodiment of thesensing system circuit and peripheral circuit.

In FIG. 17, m1 a to m255 d are sensing system circuits shown in FIG. 1.Four sensing system circuits are connected to a pair of global bit linesGBL[ ], /GBL[ ]. For example, m1 a to m1 d are connected to the GBL[0]and GBL[1]. (It means that a degeneration degree is 4.) 1001 designatesa word decoder, 1002, a control circuit for signal line such as RBK and1003, a word line potential detecting circuit.

Only one of the word lines WL[0] to WL[255] is driven with the worddecoder 1001. Simultaneously, the dummy word line WL_D is driven todetect that the dummy word line is asserted with the detecting circuit1003 a. Although not particularly restricted, a structure of thedetecting circuit 1003 a may be realized by adjusting the thresholdvalue of an ordinary inverter. Assertion of the word line is detectedand the CSN is also asserted. Thereby, the sense amplifier is activatedand any one of the local bit lines LBL[0], /LBL[n] precharged to thevoltage VDD is driven to 0V. Potential change of a pair of the local bitlines is detected with a sense end detecting circuit 1002 a within thecontrol circuit. Thereafter, the RBK is asserted to execute the re-writeoperation to the memory cell.

For example, the read operation of memory from the assertion of wordline can be executed perfectly in any timing with the embodiment of FIG.1 by amplifying the potentials of the global bit lines GBL[ ], /GBL[ ]with the circuit not shown in FIG. 17 simultaneously with the assertionof RBK of the re-write operation.

In order to detect the end of amplification of the local bit lines ofthe sense amplifier, a potential difference of the bit line pair must bedetected in the case of the half VDD precharge system of the prior artand therefore the circuit structure is complicated because it isdifficult to detect such potential difference with a logic gate such asthe simple NAND gate or the like. Meanwhile, in the present invention,since the end of amplification of the sense amplifier can be detectedwhen any one of the local bit line pair is driven to 0V from 1V, thesense end detecting circuit 1002 a can be realized easily with a 2-inputNAND gate.

Moreover, the present invention can also provide another effect that theminimum value Vs min required for accurate operation of the senseamplifier can be set to a smaller value in comparison with the sensingsystem circuit of DRAM of the prior art for the bit line potentialdifference Vs which is read on the bit line from the memory cell afterthe assertion of the word line. Therefore, operation voltage can belowered, structure of the capacitor in the memory cell can be simplifiedand the manufacturing process can also be simplified.

In usual, due to the fluctuation of characteristics of the MOStransistors in the sense amplifier and unbalance of capacitance of thebit line pair in the complementary relationship, a certain Vs, forexample, about 150 mV is necessary to accurately read the memory cellinformation by activating the sense amplifier.

In the prior art, a diffusion current of the MOS transistor can be usedas an activation current of the sense amplifier immediately after thesense amplifier is activated, but a drive current resulting from thedrift current may also be used in the present invention. In general, thediffusion current largely depends on the threshold voltage and changesto a large extent depending on fluctuation of manufacturing process. Onthe other hand, fluctuation of the drift current is rather small.Therefore, the amplification operation which is not sensible tofluctuation of characteristic fluctuation of MOS transistors in thesense amplifier can be realized in the VDD precharge system.

Moreover, in the sensing system circuit of the present invention, thelocal bit lines connected to the sense amplifier is rather short and theparasitic capacitance added to its local bit lines is also small.Therefore, unbalance of capacitance added to the local bit lines issmall and therefore does not easily apply an influence on the operationof sense amplifier.

From above explanation, the sensing system circuit of the presentinvention is capable of conducting the sufficiently accurate readoperation with the potential difference of bit lines Vs which is smallerthan the minimum Vs (Vs min) required by the sensing system circuit ofthe prior art.

Embodiment 8

Next, an embodiment of the present invention of the re-write method willbe explained with reference to FIG. 22 to FIG. 26. FIG. 22 shows anordinary embodiment modified from embodiment shown in FIG. 1, FIG. 5,FIG. 7, FIG. 10, FIG. 13, FIG. 14, FIG. 15, FIG. 16 and this embodimentindicates a relationship between the sense amplifier circuit and memoryarray designated as SAMPa, SAMPb. Here, the precharge circuit is omittedto simplify the drawing W1[1] to WL[m] indicate the word lines and amemory cell MC is connected to an intersection with the bit line in theconnecting mode shown in the figure. The sensing system circuit such asthe sense amplifier circuit or the like is connected in zig-zag at oneend of the bit lines as shown in the figure. Although it goes withoutsaying, the signals /SH of FIG. 10, /SH_UP and /SH_DW of FIG. 16correspond to the RBL in FIG. 22. Moreover, the signal CSP of FIG. 10 iseliminated in FIG. 22 but such signal is represented by the CSN in FIG.22.

FIG. 23 is a timing chart showing the re-write operation method of FIG.22. However, in view of avoiding the duplicated explanation, only thewaveforms after a certain period has passed from assertion of the wordlines and also assertion of the activate signal of the sense amplifierare shown in this figure (Time T2′ of FIG. 23 corresponds, for example,to the time T2′ of FIG. 2.) Moreover, in the explanation of operationsof the embodiments shown in FIG. 1, FIG. 5, FIG. 7, FIG. 10, FIG. 13,FIG. 14, FIG. 15, FIG. 16, it has been assumed that the YS[k] is alreadyasserted when the sense amplifier is activated, but here YS[k] isasserted at the time T2 a after the re-write operation (drive of theBL[n], /BL[n] of the sense amplifier) by the assertion of RBK at thetime T2′. Since the YS[k] is asserted at the time T2 a, the local bitlines LBL[n], /LBL[n] selected with the Y switch are connected to theglobal bit lines GBL [p], /GBL[p] and only one of the global bit linesGBL[p], /GBL[p] precharged to the potential VDD is driven to 0V.

In the re-write method of FIG. 23, both M206 a and M207 a aresimultaneously turned ON with assertion of the RBK. Therefore, as shownin FIG. 23, the local bit lines LBL[n], /LBL[n] are respectively chargedor discharged up to the potentials indicated as V1 and V2 with thecharge sharing and thereafter are respectively charged or discharged to1.0V and 0V with the drive by the sense amplifier. For example, as shownin FIG. 1, an input voltage of the inverter circuit (consisting of M200and M202 in the embodiment of FIG. 1) to drive the local bit line LBL[n]in the sense amplifier circuit is set to /LBL[n] and the input voltageof the inverter circuit (consisting of M201 and M203 in the embodimentof FIG. 1) to drive the local bit line /LBL[n] is set to LBL[n].Therefore, since the input voltage of the inverter to be driven becomesan intermediate voltage (V1, V2) as explained above, a drive current ofthe inverter to drive the local bit lines LBL [n], /LBL [n] is loweredand thereby the time (tRBK) required to charge or discharge the localbit lines LBL[n], /LBL[n] to 1.0V and 0V becomes longer.

FIG. 24 is a diagram showing an embodiment to solve the problemsexplained above. Here, the precharge circuit is also omitted to simplifythe drawings. In comparison with FIG. 22, the gate terminals of a pairof the MOS transistors for re-write operation connected between a pairof bit lines and a pair of local bit lines are controlled with differentwrite back signals RBK1, RBK2. Moreover, the global bit lines areisolated to the global bit lines GBLR[p] and /GBLR[p] for read operation(the third bit line pair) and the global bit lines GBLW[p] and /GBLW[p]for write operation (the fourth bit line pair) and the global bit linesGBLR[p], /GBLR[p] for read operation are connected to the local bitlines LBL[n], /LBL[n] with the PMOS transistors indicated by M150 a,M151 a, M150 b, M150 b. On the other hand, the global bit lines GBLW[p],/GBLW[p] for write operation are connected to the local bit linesLBL[n], /LBL[n] with the NMOS transistors indicated by M107 a, M108 a,M107 b, M108 b. Although not illustrated in the drawings, the global bitlines GBLR[p], /GBLR[p] for read operation are precharged to the VDDvoltage (second precharge voltage) with the precharge circuit.

A structure of the global bit lines can, of course, be used independentof the re-write method of the present invention. The effect obtained bysimultaneously using these global bit lines and the re-write method ofthe present invention can further be improved and therefore only theembodiment when these are used simultaneously will be explained.

FIG. 25 is a diagram showing the operation waveforms of the re-writeoperation method of the present invention as the embodiment of FIG. 24.Similar to FIG. 23, the waveforms after a while when the word line isasserted and then the activate signal of the sense amplifier is alsoasserted are shown. (The time T2′ of FIG. 23 corresponds, for example,to the time T2′ of FIG. 2.) In FIG. 25, only one of the two write backsignals RBK1, RBK2 is asserted during the re-write operation at the timeT2′. Namely, only the MOS transistor for re-write operation connected tothe bit lines which are connected via the memory cell to the assertedword line among the two MOS transistors for re-write operation indicatedas M206 and M207 is turned ON (M206 a, M206 b of FIG. 24 in the exampleof FIG. 25). When the word line to be asserted is determined, to whichbit line among the bit line pair BL[n], /BL[n] the memory cell connectedto the word line is connected is also determined uniquely. Therefore,the MOS transistor for re-write operation to be turned ON is of coursedetermined depending on such determination. For example, in FIG. 24,when the word lines WL[2], WL[3], WL[M−1], WL[m] are asserted, it isenough that the M206 a, M206 b are turned ON (conductive condition) andwhen the word lines WL[0], WL[1], WL[M−3], WL[M−2] are asserted, it isenough that the M207 a, M207 b are turned ON.

Thereby, during the re-write operation, only the bit line (BL[n] in theexample of FIG. 25) is connected to the corresponding local bit line(LBL[n] in the example of FIG. 25) and the complementary bit line(/BL[n] in the example of FIG. 25) is not connected to the correspondinglocal bit line (/LBL[n] in the example of FIG. 25). Therefore, chargesharing during the re-write operation is generated only on the bit line(BL[n] in the example of FIG. 25) and the local bit line (LBL[n] in theexample of FIG. 25). Accordingly, during the re-write operation, thepower source voltage or the ground potential itself is applied as aninput voltage of the inverter circuit within the sense amplifier todrive the bit line (BL[n] in the example of FIG. 25) and the local bitline (LBL[n] in the example of FIG. 25).

Therefore, a drive current of the bit line (BL[n] in the example of FIG.25) and local bit line (LBL[n] in the example of FIG. 25) during there-write operation becomes larger than that in the method of FIG. 22 andFIG. 23. As a result, the time tRBK required for re-write operation canbe shortened. Moreover, since the re-write operation time has a delaytime characteristic equal to a delay time of the inverter, there rises amerit that such delay time is well matched with the delay time of thelogic circuit.

Moreover, in the case where the YS[k] is asserted after the re-writeoperation as shown in FIG. 23, when the time tRBK required for re-writeoperation is shortened, the time (period up to the time T2′ from thetime T2 a) required until the YS[k] is asserted can also be shortened.In addition, the bit lines BL[n], /BL[n] are connected with a heavierload and a large amount of power is required to charge and dischargethese bit lines. Since the re-write operation can be realized only withthe drive of one bit line with this system, power consumption requiredfor charge and discharge of these bit lines can also be lowered.

FIG. 26 shows another embodiment of the re-write operation methoddifferent from that of FIG. 25. Similar to FIG. 25, the waveforms aftera while from assertion of the word line and also assertion of theactivate signal of the sense amplifier are illustrated. (The time T2′ inFIG. 23 corresponds, for example, to the time T2′ in FIG. 2.) As in thecase of FIG. 25, when the re-write operation is conducted at the timeT2′, only one signal of the two write back signals RBK1, RBK2 isasserted. Thereafter, at the time T2 b, the remaining one signal of twowrite back signals RBK1, RBK2 is asserted. The re-write operation iscompleted after tRBK of the time T2′ and high speed re-write operationcan be conducted as in the case of FIG. 25.

The method of the embodiment shown in FIG. 25 has a disadvantage that aload is generated in the power supply circuit to supply the potentialVBM because a sum of the potential of the bit line BL[n] and a potentialof the bit line /BL[n] is not equal to a potential (VBM) which is only ahalf of the array voltage when the bit lines BL[n], /BL[n] areprecharged. Meanwhile, the method of the embodiment shown in FIG. 26 hasan advantage that such problem can be eliminated because a sum of apotential of bit line BL[n] and a potential of bit line /BL[n] becomesequal to a half of the voltage (VBM) of the array voltage when the bitlines BL[n], /BL[n] are precharged. Therefore, it is recommended toselect the re-write operation method of FIG. 25 and FIG. 26 depending onthe capacitance of the VBM power supply and application field of thedynamic memory of the present invention.

The re-write operation method of the present invention shown in FIG. 25and FIG. 26 is not particularly restricted in the application field onlyto the sensing system circuit shown in FIG. 22. For example, it isenough when the output terminal pair of the sense amplifier circuit (n3,n4 of SAMPa of FIG. 22) is connected to the bit line pair connected tothe memory cell (BL[n], /BL[n] of FIG. 22) via the source and drainroutes of a pair of MOS transistors (M206 a, M207 a of FIG. 22).

Moreover, it is of course possible to apply the present invention to thesensing system circuit of an ordinary DRAM described, for example, inthe [Reference 1].

Embodiment 9

In above embodiments, the address supply system is not particularlyrestricted. But, the embodiment of the present invention shown in FIG.24 is used for a dynamic memory to which an address is supplied withoutmultiplexing (the row address and column address, bank address, etc. aresupplied simultaneously).

First, an embodiment of the timing chart of the read operation is shownin FIG. 27. Here, an operation example will be explained based on thesense amplifier circuit of FIG. 7 as an example thereof indicated asSAMPa and SAMPb of FIG. 24 in order to simplify the explanation. Thepart same as that of FIG. 8 is not explained here to eliminateduplication of explanation.

In FIG. 24, the global bit line is isolated to the global bit lines forread operation GBLR[p], /GBLR[p] and the global bit lines for writeoperation GBLW[p], /GBLW[p]. Therefore, the YS[k] is being negatedduring the read operation. At the time T2, the sense amplifier isactivated. When the local bit lines LBL[n], /LBL[n] are driven to 1.0Vand 0V, any one (M151 a in the example of FIG. 27) of the PMOStransistors M150 a and M151 a is turned with the local bit line (/LBL[n]in the example of FIG. 27) driven to 0V. Thereby, any one (/GBLR[p] inthe example of FIG. 27) of the global bit lines for read operationGBLR[p], /GBLR[p] precharged to the potential VDD is discharged.Moreover, as the re-write operation method, the method of FIG. 25 isintroduced and only one of the two write back signals RBK1, RBK2 isasserted at the time T2′. Namely, only the MOS transistor for re-writeoperation connected to the bit lines connected to the asserted word linevia the memory cell among the two MOS transistors for re-write operationindicated as M206 and M207 is turned ON. (M206 a, M206 b of FIG. 24 inthe example of FIG. 27)

With the control method explained above,

(G1) when the local bit lines LBL[n], /LBL[n] are amplified at a highspeed with the precharge system of the present invention, the global bitlines GBLR[p], /GBLR[p] for the read operation are amplifiedcontinuously in any timing and accordingly the high speed read operationof memory cell information becomes possible, and

(G2) since the re-write operation is completed at a high speed, the timerequired until the word line is asserted and is then negated can beshortened and accordingly a higher pipeline frequency can be realizedwhen the dynamic memory using this sensing system circuit is formed asthe pipeline memory.

A circuit structure connecting the local bit lines LBL[n], /LBL[n] andthe global bit lines for read operation GBLr[n], /GBLr[n] is notparticularly restricted to the circuit structure consisting of the PMOStransistors M150 and M151 of FIG. 24. For example, the PMOS transistorsmay be replaced with the NMOS transistors. However, in this case, thelocal bit lines LBL[n], /LBL[n] are not connected in direct to the gateterminal of the NMOS transistor, but these local bit lines LBL[n],/LBL[n] are connected to the gate terminal via the inverter circuit. Incomparison with the embodiment of FIG. 24, the number of transistorsincreases as much as two inverters but since the threshold voltage Vthdoes not drop, the global bit lines GBLR[p], /GBLR[p] for read operationcan be driven at the still higher speed.

Embodiment 10

Next, an embodiment of the timing chart (an example of inverted writeoperation) of the write operation is shown in FIG. 28. Here, anoperation example will be explained based on the sense amplifier circuitshown in FIG. 7 as an example this circuit indicated as SAMPa and SAMPbof FIG. 24 in order to simplify the explanation. In order to eliminateduplication of explanation, explanation of the part same as that of FIG.8 is omitted.

Since an address is not multiplexed, a write data is supplied togetherwith the address. Therefore, the global bit lines GBLW[p], /GBL″[p] forwrite operation are driven using the write data at the time T0.Thereafter, the bit line to execute the write operation is selected andthe YS[k] is asserted at the time T1 in the same timing as the assertionof the word line depending on such selecting operation. This YS[k] isnegated simultaneously with the activation of sense amplifier (time T2).The data corresponding to the write data appears on the local bit linesLBL[n], /LBL[n] corresponding to the bit lines for write operation andthe potential difference Vs3 corresponding to the write data appearswhen the sense amplifier is driven (time T2). The sense amplifiercircuit amplifies the potential difference Vs3 to charge and dischargethe local bit lines LBL[n], /LBL[n].

Here, it is also a characteristic that the timings other than that ofthe YS[k] are same as the read timing. Therefore, for the bit lines notselected with assertion of the YS[k] during the read operation, there-write operation is carried out in the same time as the readoperation. Moreover, in the ordinary write system to the DRAM, the writeoperation is executed to the memory cell after the re-write operationresulting from the read operation. However, in the method of the presentinvention explained above, the write operation and re-write operationare conducted in parallel. Therefore, the period until negation fromassertion of the word line can be shortened. When the dynamic memoryusing the sensing system circuit is formed as the pipeline memory, thepipeline frequency thereof can be increased. Moreover, in the embodimentexplained above, the write data inputted to the global bit linesGBLW[p], /GBLW[p] for write operation is amplified with the senseamplifier precharged to the voltage VDD and the amplified data iswritten into the memory cell with the re-write method of the presentinvention. Therefore, it is also a characteristic that the time requiredfor the write operation is well matched with a delay time of the logiccircuit.

In the embodiment of FIG. 28, the assertion of YS[k] is conductedsimultaneously with the assertion of the word line and the YS[k] isnegated simultaneously with the activation timing of the senseamplifier, but the present invention is not restricted only to thistiming. Timing for asserting the YS[k] is preferably set to the timingthat a potential difference Vs3 is generated on the local bit linesLBL[n], /LBL[n] depending on the write data when the sense amplifier isactivated. Moreover, timing for negating the YS[k] is preferably set insuch a manner that any problem is not generated for the precharging ofthe local bit lines LBL[n], /LBL[n].

Here, it is of course possible that the sense end detection circuitshown in FIG. 17 is used for generation of the timing for assertion ofthe write back signal in the re-write method shown in FIG. 25 to FIG.28.

The read and write methods using the global bit line of the presentinvention shown in FIG. 27 and FIG. 28 are not particularly restrictedin the application thereof only to the sensing system circuit shown inFIG. 24. For example, when the sense amplifier is used by the VDDprecharge system even in the sensing circuit of the ordinary DRAMdescribed in the [Reference 1], the global bit lines are isolated to thelines for read operation and the lines for write operation. Therefore,these global bit lines can be used in the same manner and the similareffect can also be attained by adding the read amplifier circuitcorresponding to the PMOS transistors M150 a, M151 a of FIG. 24 to theglobal bit lines for read operation.

Connections of the substrate potential of the MOS transistors are notobviously described in the drawings of the embodiments shown in FIG. 1to FIG. 28, but the connecting methods are not particularly restricted.Moreover, in the embodiments shown in FIG. 1 to FIG. 28, a destructivecell which requires re-write (DRAM cell of the so-called 1T1C typeformed of one transistor and one capacitor) is assumed but it is ofcourse possible to introduce the method of the present invention, forexample, to the sensing system circuit of the memory array including thenon-destructive read cell consisting of three NMOS transistors.Particularly, the structure of memory cell is not restricted. In theembodiments of the present invention, the power source potential isassumed to a certain value under the condition that the bit lineamplitude is 1.0V and setup voltage of the word line is 2.5V or the likefor the explanation but the present invention is not of courserestricted thereto.

The major effects of the present invention are as follows.

(1) By using the power source voltage of sense time, re-write time andwrite time respectively can be set almost to the characteristic that issame as the sense time characteristic of the VDD precharge system shownin FIG. 21. Namely, the sense time becomes further even in the lowvoltage operation when temperature is lower in comparison with the sensetime when temperature is high and deterioration of sense rate in the lowvoltage operation can be maintained to the similar degree as the delaytime of the CMOS inverter shown in FIG. 20(B). With this characteristic,the DRAM macro using the sensing system circuit of the present inventionis provided with the characteristic well matched with the low voltagecharacteristic of the logic circuit. Therefore, it can no longer beconcluded that any one of the logic circuit and the DRAM macro mainlycontrols the low voltage characteristic and thereby the DRAM macro andlogic LSI can be mounted on the same substrate without considerabledeterioration of the final LSI characteristic.

(2) The temperature characteristic of the DRAM macro is same as that ofthe logic circuit because a drive current of the sense amplifier of thepresent invention is mainly governed with a drift current in place of adiffusion current of the drain current of MOS transistors. In general,the diffusion current changes very sensibly for temperature andthreshold voltage of MOS transistors. Therefore, when the senseamplifier is used in the region where the diffusion current governs theoperation in place of the drift current like the sensing system circuitof the prior art, the sense time changes largely for fluctuation ofmanufacturing process of LSI and fluctuation of operation environment ofLSI. This change may be developed up to a deterioration of the yieldrate of the LSI circuit and as a result, manufacturing cost of the LSIusing the DRAM circuit of such structure becomes high. Therefore, thesensing system circuit of the present invention has a characteristicthat it is resistive to fluctuation in the manufacturing process of LSIand fluctuation in the operation environment of LSI. Moreover, thecircuit of the present invention can be said to have a higher yieldrate.

(3) While the characteristic of the VDD precharge system is utilized, aparticular cell such as a dummy cell which has been required in theexisting VDD precharge system is no longer required. Therefore, themanufacturing process and circuit can remarkably be simplified toimprove the yield rate and assures low manufacturing cost of LSI.

(4) Since it can be judged that amplification of the sense amplifier iscompleted when only one of the local bit line pair is driven to 0V inorder to detect that the amplification of the local bit line of senseamplifier is completed, a sense end detection circuit may be easilyformed with a 2-input NAND gate and the read operation can be executedin any timing.

(5) The minimum value required for accurate operation of the senseamplifier for a potential difference Vs of the bit line read on the bitlines from the memory cell after the assertion of the word line can beset to a small value in comparison with a value of the existing sensingsystem circuit.

INDUSTRIAL FIELD OF UTILIZATION

The present invention can be used as a sense amplifier for detecting andholding the signals and particularly suitable for detection ofinformation stored in a memory cell consisting of one MOSFET and onecapacitor. The present invention can also be applied to a DRAM such as asingle DRAM, DDR-SDRM and DRAM including the other circuits.

1-22. (canceled)
 23. A semiconductor device, comprising: first andsecond bit lines; a plurality of memory cells coupled to the first andsecond bit lines; third and fourth bit lines; a first transistor coupledbetween the first bit line and the third bit line; a second transistorcoupled between the second bit line and the fourth bit line; a senseamplifier including a first circuit coupled between the first bit lineand the third bit line and between the second bit line and the fourthbit line, and a second circuit coupled between the third bit line andthe fourth bit line; a first precharge circuit which precharges thefirst and second bit lines to a first precharge potential; and a secondprecharge circuit which precharges the third and fourth bit lines to asecond precharge potential, wherein said second circuit amplifies asignal on one of the third and fourth bit lines to a first potential anda signal on the other of the third and fourth bit line to a secondpotential according to a storage signal from a selected one of theplurality of memory cells, the storage signal being transferred to thesense amplifier via the first circuit, wherein the first prechargepotential is between the first and second potentials, wherein the secondprecharge potential is higher than the first precharge potential, andwherein a gate oxide film of the first transistor and a gate oxide filmof the second transistor are thicker than a gate oxide film of atransistor included in the second circuit.
 24. A semiconductor deviceaccording to claim 23, wherein the first transistor transfers the signalamplified by the second circuit from the third bit line to the first bitline, and the second transistor transfers the signal amplified by thesecond circuit from the fourth bit line to the second bit line.
 25. Asemiconductor device according to claim 24, wherein the first circuitincludes a third transistor having a gate connected to the first bitline and a fourth transistor having a gate connected to the second bitline and a source connected to a source of the third MOSFET, and whereinthe second circuit includes a latch circuit.
 26. A semiconductor deviceaccording to claim 23, wherein the first precharge circuit is coupledbetween the first and second bit lines.
 27. A semiconductor devicecomprising: first and second bit lines coupled to a plurality of memorycells; a third bit line coupled to the first bit line; a fourth bit linecoupled to the second bit line; a pre-sense-amplifier which generates adifference of potential between the third and fourth bit lines accordingto a difference of potential between the first and second bit lines; asense amplifier coupled between the third and fourth bit lines andincluding PMOS transistors which are supplied with a first potential andNMOS transistors which are connected a first control line whichtransfers a first control signal; a first pre-charge circuit whichprecharges the first and second bit lines to a first pre-chargepotential; a second pre-charge circuit which precharges the third andfourth bit lines to a second pre-charge potential higher than the firstpre-charge potential; and a second control line which transfers a secondcontrol signal by which an operation state of the pre-sense amplifier iscontrolled, wherein the pre-sense amplifier starts to generate thedifference of potential between the third and fourth bit lines bydischarging the third and fourth bit lines when the second controlsignal is activated, wherein the PMOS transistors of the sense amplifierstart to amplify the difference of potential between the third andfourth bit lines when the second control signal is activated, whereinthe NMOS transistors of the sense amplifier start to amplify thedifference of potential between the third and fourth bit lines when thefirst control signal is activated, the first control signal beingactivated after the second control signal, wherein the sense amplifieramplifies one of the third and fourth bit lines to a first potential andthe other of the third and fourth bit lines to a second potentialaccording to the difference of potential between the third and fourthbit lines, and wherein the first pre-charge potential is between thefirst potential and the second potential.
 28. A semiconductor deviceaccording to claim 27, wherein the pre-sense amplifier includes a firsttransistor and a second transistor, wherein the first transistor hassource and drain regions coupled between the second control line and thethird bit line and a gate electrode coupled to the first bit line, andwherein the second transistor has source and drain regions coupledbetween the second control line and the fourth bit line and a gateelectrode coupled to the second bit line.
 29. A semiconductor deviceaccording to claim 27, further comprising: a plurality of word linescrossing the first and second bit lines; and a switch circuit coupledbetween the first and third bit lines and between the second and fourthbit lines; wherein when one of the plurality of word lines is selected,data read out from one of the plurality of memory cells is amplified bythe sense amplifier and then the amplified data by the sense amplifieris written back to the one of the plurality of memory cells via theswitch circuit.
 30. A semiconductor device according to claim 27,wherein the second pre-charge potential is the same as the firstpotential.
 31. A semiconductor device according to claim 27, wherein thepre-sense amplifier is directly coupled to the sense amplifier.
 32. Asemiconductor device according to claim 27, wherein all of the pluralityof memory cells are dynamic memory cells.